The structure of potassium tetrahydrogen pentafluoride

The compound KF·4HF, prepared by the addition of anhydrous hydrofluoric acid to potassium fluoride, is shown, on the basis of an X-ray structure determination, to be potassium tetrahydrogen pentafluoride, K[H₄F₅]. The material crystallizes with four formula units in the tetragonal space group C_4h⁶-I4₁ a in a cell of dimensions a = 6.384(3), c = 13.227(7) Å. The conventional R factor for the 221 intensities observed above background is 6.2%. The structure is of the scheelite type, with the potassium and the central fluorine atoms residing at special positions of symmetry 4, and the other fluorine atoms hydrogen bonded to the central fluorine atom at an FH···F distance of 2.453(2) Å. The resultant H₄F₅^- ion, which is crystallographically required to possess 4 symmetry, exhibits a slight but significant distortion from tetrahedral geometry. The FH···F bond is approximately linear and is unsymmetric with the hydrogen closer to the peripheral fluorine. The fluorine atoms in the immediate environment of the potassium atom form a dodecahedron with triangular faces.